While both thermoplastic materials and elastomers are widely used in industrial and domestic applications, certain "hybrid" blended polymers displaying the attributes of both have come to be recognized as more desirable than either in certain applications, particularly in the automotive field. These blended polymers are generically known as thermoplastic elastomers.
A group of these materials, sometimes termed polyolefinic thermoplastic elastomers, POTPE, avoids the disadvantage of requiring rubber machinery for processing, but may be processed instead on conventional plastic processing equipment. Thus, while the materials display rubber-like properties at room temperature, they become plastic at elevated temperatures. This characteristic, for example, allows the blended materials to be readily extruded, injection molded, and otherwise worked without any need for excessive cycle times, specialized machinery, or other costly and often inconvenient processing expedients.
Among other applications, polyolefinic thermoplastic elastomers have been widely used in the form of foamed materials. Such products, for instance, are readily foamed during the process of extrusion by being injected with chlorofluorocarbons such as, for example, Freon 11, marketed by the EI DuPont Company of Wilmington, Del., and foamed products exhibiting desirably low densities, that is, in the order of 0.4 gms. per cc, have been formed in this way.
The use of chlorofluorocarbons has been linked by some with damage to the earth's ozone layer, however, and attempts have been made to find substitutes for such blowing agents. In this connection, modified forms of chlorofluorocarbons, i.e., hydrochlorofluorocarbons, HCFCs, have been suggested as possible substitutes for chlorofluorocarbons; however, these materials are still under development and their usefulness as replacement materials is still not certain. Furthermore, injection techniques require elaborate apparatus and necessitate careful monitoring, thus they are burdensome for those reasons as well.
Another approach to foaming has involved the use of so-called "chemical blowing agents", i.e., substances that decompose upon being heated to their decomposition temperature, producing desired blowing gases, such as nitrogen. Such materials pose no threat to the environment. However, when used with polyolefinic thermoplastic elastomers such as, for example, Santoprene, a POTPE marketed by Monsanto Company of Saint Louis, Missouri, the chemical blowing agents appear unable to produce foamed products with densities as low as those achievable with the chlorofluorocarbons. In this regard, they appear able only to effect marginal density reductions, that is, in the order of 15% to 20%. In any event, there is considerable potential advantage in finding suitable techniques for blowing polyolefinic thermoplastic elastomers without employing chlorofluorocarbons, but in a way in which blends exhibiting desirably low densities can be achieved. Such advantage includes the fact that POTPEs can be processed like thermoplastics, while exhibiting the resilience and elasticity of elastomers. In the automotive field, for example, such materials lend themselves to uses including weather stripping for automobiles, due to the excellent sealing ability made possible as a consequence of their softness.